For providing pigments with improved dispersibility, it has conventionally been practiced to treat pigment particle surfaces by various methods. Examples include the treatment with a low-molecular compound such as rosin, the adsorption treatment with a surfactant, the coating treatment with a resin, and so on. Among these, the surface treatment of pigment particles with a resin has been studied. It is practiced, for example, to knead a pigment in the presence of a resin, to allow a pigment to precipitate subsequent to its dispersion in the presence of a resin, or to encapsulate a pigment with a resin by making use of a reaction between a resin and particle surfaces of the pigment. In the case of the encapsulation of the pigment with the resin by making use of the above-described reaction, it is required that three-dimensional crosslinking is induced through the reaction to enclose the pigment with the resin, and that the water-insoluble, and moreover, encapsulated pigment is dispersed in a solvent to form a pigment dispersion of good quality.
In particular, an inorganic pigment is high in specific gravity because of the inclusion of a metal element as its constituent. When dispersed, the inorganic pigment hence settles under gravity and the settlings aggregate hard or, even when allowed to disperse, the inorganic pigment flocculates into larger particle sizes so that the resulting dispersion cannot maintain dispersion stability as it is. With a view to overcoming these difficulties, it has been practiced to use various pigment dispersants developed for inorganic pigments or to treat pigment particles at surfaces thereof to maintain the dispersibility. As a surface treatment method for the above-mentioned approach, inorganic treatment such as silica-alumina treatment, treatment with a silane coupling agent, or resin treatment is practiced.
As methods for treating the surfaces of pigment particles with a resin, there are the following methods:    [1] After the pigment is dispersed with a resin dispersant or the like by the above-described method, the dispersed pigment is allowed to precipitate such that the pigment particles are coated at the surfaces thereof with the resin.    [2] Two kinds of compounds, which have groups reactable with each other, respectively, or a compound or resin, which has mutually-reactable groups in a molecule thereof, are added, and these reactable groups are allowed to react with each other on the surfaces of the pigment particles to form three-dimensional linkages such that the pigment particles are encapsulated with the resultant, solvent-insoluble resin.    [3] The resin and the pigment (especially, an inorganic pigment) are chemically bonded with each other by the below-described procedure. In this case, a compound having a group reactable to the surfaces of inorganic pigment particles, that is, an alkoxylsilyl group, titanate group, zirconia group or the like and a functional group such as a thiol group, amino group or glycidyl group is first allowed to react with the surfaces of the pigment particles or the surfaces of pigment particles surface-treated with an inorganic compound introduced beforehand. Using a resin having groups reactable with functional groups such as the amino groups, the functional groups and the resin are next reacted with each other to coat the pigment particles with the resin.    [4] A resin having functional groups such as alkoxysilyl groups is allowed to react with the surfaces of the pigment particles or the pigment particles surface-treated with an inorganic compound as a surface treatment such that the pigment particles are treated at the surfaces thereof with the resin.    [5] A silane coupling agent or the like, which has a functional group, for example, a vinyl group, (meth)acryloyloxy group or thiol group, that takes part in polymerization, is reacted to the surfaces of the pigment particles, and polymerization is then conducted in the presence of a monomer to introduce the functional groups in the resultant polymer.
Recently, there is also a method such as that to be described below.    [6] In living radical polymerization, initiation groups for the living radical polymerization, for example, bromoisovaleric acid is introduced, and the living radical polymerization is conducted to bond the terminals of the resultant polymer with the surfaces of the pigment particles.
On the other hand, processes, which make use of living radical polymerization, have been developed in recent years for the production of block copolymers, including polymerization processes that permit easy control of structure and molecular weight. Specific examples include those to be described below, and a wide variety of research and development activities are now underway.
The nitroxide mediated polymerization (NMP) process that makes use of dissociation and bonding of amine oxide radicals (see Non-patent Document 1).
The atom transfer radical polymerization (ATRP) process that conducts polymerization in the presence of a halogen compound as an initiating compound by using a heavy metal such as copper, ruthenium, nickel or iron and a ligand capable of forming a complex with the heavy metal (see Patent Document 1, Patent Document 2, and Non-patent Document 2).
The reversible addition-fragmentation chain transfer (RAFT) process that conducts polymerization by using an addition-polymerizable monomer and a radical polymerization initiator in the presence of a dithiocarboxylate ester, a xanthate compound or the like as an initiating compound (see Patent Document 3), and the macromolecular design via interchange of xanthate (MADIX) process (see Patent Document 4).
The degenerative transfer (DT) process that makes use of a heavy metal such as an organotellurium compound, organobismuth compound, organoantimony compound, antimony halide, organogermanium compound or germanium halide (see Patent Document 5 and Non-patent Document 3).